A utility company supplies potable water from a water treatment plant through a distribution system of underground mains pipes of reducing diameter and finally through a relatively small service pipe to individual premises.
The connection between the service pipe and the mains pipe is usually through a fitting known as a ferrule. This is a fitting which is screwed into an aperture in the mains pipe and is then connected to the service pipe. The ferrule includes a shut-off valve, so that it can be installed into the mains pipe, and the service pipe connected, without shutting off the flow through the mains pipe. The shut-off valve isolates the service pipe from the mains pipe, and so enables work to be carried out on the service pipe without shutting off the flow through the mains pipe. A utility company will try to avoid shutting off flow through a mains pipe, because of disruption to customers. It is also costly, as a shut off requires a considerable amount of time to organise and implement, and may involve the need to test the water quality before flow is restored.
The shut-off valve may fail to operate when needed, usually due to corrosion. It is also possible for a leak to occur at the connection of the ferrule to the mains pipe. Either of these will require the flow to the mains pipe to be shut off, either to enable the ferrule to be replaced, or the leak to be sealed.